Java - write opcodes to .class file (for a custom compiler)

Question

I played a bit around with Compilers. And I want to compile my language into JVM Bytecode but I was wondering how I could write those Opcodes to a .class file.

Example:

I have my .atan src file and I want to compile this to a .class file. I can interpret this code but I do not know how to write those opcodes to the .class file. How can this be done?